1. Technical Field
The present invention relates to the field of single-phase induction motors, and more particularly to a switching device for electrically connecting and removing the starting winding from the single-phase induction motor's circuitry.
2. Background Information
Single-phase induction motors, as are well known to those skilled in the art, typically comprise a distributed stator main winding, an auxiliary winding, and a squirrel-cage rotor. As discussed in the Del Toro's test Electromechanical Devices For Energy Conversion And Control Systems, Prentice-Hall, Inc. 1968, pps. 360-90, an a.c. supply voltage applied only to the stator winding creates a field fixed in space and alternating in magnitude. The field therefore produces no starting torque on the rotor. This condition, however, prevails only at rotor standstill. If, by some means, the rotor is started in either direction, it will develop a nonzero net torque in that direction and thereby cause the motor to achieve synchronous speed.
The typical non-mechanical method of starting a single phase induction motor is to temporarily include a second, auxiliary winding around the rotor to produce a revolving field of constant amplitude and constant linear velocity. This revolving field creates the necessary starting torque needed to start the rotor turning on its axis. To obtain this revolving field, the two windings are preferably space-displaced by 90 electrical degrees. Additionally, the current flowing through these windings are preferably time-displaced by 90 electrical degrees and the windings must have such magnitudes that the mmf's are equal.
The space-displacement criterion is met by placing the auxiliary winding in the stator with its axis in quadrature with that of the main winding. Typically, the main winding occupies two-thirds of the stator slots, with the auxiliary winding occupying the remaining one-third.
The time displacement criterion regarding the currents through the two windings is at least partially obtained by designing the auxiliary winding for high resistance and low leakage reactance. This is in contrast to the main winding which typically has lower resistance and higher leakage reactance. Due to the high-resistance characteristic and the short time power rating inherent in the auxiliary winding, they must be removed from the line once a sufficient percentage of synchronous speed is reached.
One prior art method for removing the auxiliary winding from the line is by a cut-off switch, placed in the auxiliary winding circuit, which by centrifugal action electrically removes the auxiliary winding from the line when the motor speed obtains a certain percentage of synchronous speed. However, due to the large current flow and the switching action, the centrifugal switch contacts become damaged over time due to arcing. This is disadvantageous because the auxiliary winding could burn itself out if the switch becomes faulty. Additionally, since the switch resides on the motor, it is difficult to miniaturize the overall motor size.
Another prior art method for removing the auxiliary winding from the line involves replacing the centrifugal action switch with a triac-based circuit, wherein the triac is controlled based on the current in the main stator winding. However, because the current which flows through the main winding changes with load characteristics, this method is complex and inherently limited to a small range of applications.